(1) Field of the Invention
The present invention relates to a radio frequency (RF) power amplifier used for transmission in mobile communications such as cellular phone terminals.
(2) Description of the Related Art
To realize a light, compact cellular phone terminal which allows long-time communication, it is considered to be important to increase efficiency (decrease power consumption) of a transmission power amplifier which consumes a comparatively large amount of power, in addition to downsizing batteries. A transmission power amplifier for the cellular phone terminal is called a power amplifier (PA) module, for which a GaAs radio frequency transistor which excels in radio frequency characteristics and power conversion efficiency is mainly used. The GaAs RF transistor is roughly classified into: a field effect transistor (hereinafter described as FET) and a heterojunction bipolar transistor (hereinafter described as HBT).
In the CDMA schemes represented by wideband-code division multiple access (W-CDMA), a technique of controlling RF power that is output from an antenna of the mobile terminal is used according to a distance to a base station or surroundings such that the RF power arriving at each base station is almost equal. Generally, the level of output from the antenna is higher when the distance to the base station is longer, and the level of output from the antenna is lower when the distance is shorter. The output from the antenna is controlled by controlling the output level from the PA module. The output from the antenna, when used, is often suppressed to a relatively low level, and it is extremely important to realize increase in efficiency of the PA module under low output conditions in order to reduce power consumption. However, the power amplifier is designed to have highest power efficiency when the output level is high. Therefore, the power efficiency decreases when the level of output is less than the high output level.
As a means to improve efficiency at the time of middle output and low output, a PA module is suggested which switches an output path used for the output power that is required (Patent Reference 1: US Patent Application Publication No. 2007/0222523, DESCRIPTION). FIG. 7 is a block diagram of the PA module shown in Patent Reference 1.
Hereinafter, a conventional PA module will be described with reference to the diagram. Note that in the description below, the same numerical references will be assigned to the same elements.
First, an operation of a high power output path is described. An RF power, which is input from an input terminal 721, passes through an input matching circuit 711, to be input into a high output amplifier 701. The RF power, amplified by the high output amplifier 701, passes through the output matching circuit 714, to be output from an output terminal 722.
Next an operation of a middle power output path is described. An RF power, which is input from the input terminal 721, passes through the input matching circuit 712, to be input into a middle output amplifier 702. The RF power, amplified by the middle output amplifier 702, passes through a matching circuit 715, a switch 704, and the output matching circuit 714 in order, to be output from the output terminal 722.
Next an operation of a low power output path is described. An RF power, which is input from the input terminal 721, passes through an input matching circuit 713, to be input into a low output amplifier 703. The RF power, amplified by the low output amplifier 703, passes through a matching circuit 716, a switch 705, the matching circuit 715, the switch 704, and the output matching circuit 714 in order, to be output from the output terminal 722.